This invention relates to planar index-guided oxide vertical cavity surface emitting lasers (VCSELs) and methods of making the same.
A VCSEL is a laser device formed from an optically active semiconductor layer (e.g., AlInGaAs or InGaAsP) that is sandwiched between a pair of highly reflective mirror stacks, which may be formed from layers of metallic material, dielectric material or epitaxially-grown semiconductor material. Typically, one of the mirror stacks is made less reflective than the other so that a portion of the coherent light that builds in a resonating cavity formed in the optically active semiconductor layer between the mirror stacks may be emitted from the crevice. Typically, a VCSEL emits laser light from the top or bottom surface of the resonating cavity with a relatively small beam divergence. VCSELs may be arranged in singlets, one-dimensional or two-dimensional arrays, tested on wafer, and incorporated easily into an optical transceiver module that may be coupled to a fiber optic cable.
In general, a VCSEL may be characterized as a gain-guided VCSEL or an index-guided VCSEL. An implant VCSEL is the most common commercially available gain-guided VCSEL. An implant VCSEL includes one or more high resistance implant regions for current confinement and parasitic reduction. An oxide VCSEL, on the other hand, is the most common laterally index-guided VCSEL. An oxide VCSEL includes oxide layers (and possibly implant regions) for both current and optical confinement.
VCSELs and VCSEL arrays have been successfully developed for single-mode operation and multi-mode operation at a variety of different wavelengths (e.g., 650 nm, 850 nm, 980 nm, 1300 nm and 1550 nm). The commercial success of VCSEL technology, however, will depend in large part upon development of VCSEL structures that are characterized by high performance and high reliability.
Techniques have been proposed for improving the performance and reliability of VCSELs. For example, U.S. Pat. No. 5,719,893 describes a scheme for passivating ridge and implant VCSELs against physical and chemical damage. In accordance with this scheme, a layer of insulating material covers the entire VCSEL structure, including the light-emitting aperture region and the surrounding top metal electrode. The insulating material has an optical thickness that is an integral multiple of one half of the wavelength of light that the VCSELs are designed to emit. The passivating layer covers the entire VCSEL device structure in order to protect the device from physical and chemical damage.
The invention features a scheme (systems and methods) of passivating planar index-guided VCSELs that addresses the unique susceptibility of these devices to damage that otherwise might be caused by moisture intrusion into the etch holes that are used to form the index-guiding confinement regions.
In one aspect, the invention features a VCSEL that includes a vertical stack structure having a substantially planar top surface. The vertical stack structure includes a top mirror, a bottom mirror, and a cavity region disposed between the top mirror and the bottom mirror and includes an active light generation region. At least one of the top mirror and the bottom mirror has a layer with a peripheral region that is oxidized into an electrical insulator as a result of exposure to an oxidizing agent. The vertical stack structure defines two or more etched holes each extending from the substantially planar top surface to the oxidized peripheral region. Each of the etched holes is moisture passivated by an overlying moisture penetration barrier.
Embodiments of the invention may include one or more of the following features.
The moisture penetration barrier preferably has a thickness that is selected to prevent substantial vertical moisture intrusion into the etched holes. In some embodiments, the moisture penetration barrier comprises a silicon nitride layer having a thickness of approximately 300 nm or greater.
Each of the etched holes preferably is moisture passivated by an overlying moisture penetration barrier having a lateral surface area that is sufficient to prevent substantial delamination of the moisture penetration barrier.
Multiple etched holes may be moisture passivated by a single continuous film of moisture penetration barrier material.
The VCSEL may include a top electrode that is disposed over the substantially planar top surface of the vertical stack structure and circumscribes a light emission region that is substantially free of any overlying moisture penetration barrier material. In some embodiments, the moisture penetration barrier covers a major portion of the top surface of the vertical stack structure other than the top electrode and the light emission region.
In some embodiments, the moisture penetration barrier includes a peripheral edge that intersects the top surface of the vertical stack structure at a moisture penetration interface. At the top surface of the vertical stack structure each of the etched holes may be circumscribed by a respective peripheral edge having a substantial portion that is separated from the moisture penetration interface by a distance that is sufficient to prevent substantial lateral moisture intrusion into the etched holes. For example, in some embodiments, a substantial portion of each of the etched hole peripheral edges may be separated from the moisture penetration interface by a distance of approximately 15 xcexcm or greater.
In another aspect, the invention features an array of two or more of the above-described VCSELs.
In another aspect, the invention features a method of manufacturing the above-described VCSEL.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.